RVHS 2023 JC2 H2 Prelim P3 A

RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CENTRE NUMBER S CLASS 22J INDEX NUMBER BIOLOGY 9744/03 Paper 3 Long Structured and Free-response Questions 15 September 2023 2 hours Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE ON ANY BARCODES. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 2 3 Section B Total This document consists of 23 printed pages and 1 blank page.

2 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination Section A Answer all the questions in this section. 1 The gut microbiota is the community of all micro-organisms (bacteria, archaea, protozoa, fungi, and viruses) present in the digestive tract. Bacteria are the largest component of the microbiota. (a) Describe the structure of bacterial genome. [3] 1. Haploid; 2. Double-stranded; 3. Circular DNA; 4. Associated with basic proteins; 5. Presence of plasmid(s); 6. Few million nucleotides in length; [R: introns] Gut bacteria are present in a range of species in Kingdom Animalia including insects, fish, and apes. Gut bacteria are passed vertically from mother to offspring. Its composition and abundance have changed in the generations with evolutionary time. Fig. 1.1 shows the relative change in abundance of selected bacterial taxa in the evolution of humans ( Homo sapiens ). Relative change is shown by the “_x” in the box beside the descendant species, for example “5x” refers to a five times increase in abundance from the ancestor. 19 MYA 7 MYA 2. 6 MYA Gorilla gorilla Pan paniscus Pan trogolodytes Homo sapiens 5x 2x 0.5x 0.2x 0.2x 0.2x 0.5x 0.5x 5x 5x 2x 2x Bacterial genera present in ancestor Bacteroides Lactobacillus Clostridium Fibrobacter Lactobacillus Lactobacillus Fibrobacter Fibrobacter Bacteroides Clostridium Clostridium Bacteroides Clostridium key MYA: Million years ago

3 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination Adapted from Moeller et al, PNAS, 111(46), 2014 Fig. 1.1 (b) With reference to the animals in Fig. 1.1, explain how phylogeny is related to classification. [2] 1. degree of relatedness in evolutionary relationship of phylogeny; 2. is observed in the hierarchical classification of species; 3. organisms of same genus are more closely related / diverged more recently / have a more recent common ancestor; [A: reverse argument] 4. shown by shorter distance between branches of species Pan trogolodytes and Pan paniscus; 5. and the longer distance between branches of genus such as Homo sapiens and Gorilla gorilla; Max. 2m Diet has a significant influence on gut bacteria composition. Different diets contribute different nutrients to the digestive tract which affects the presence and proportion of various bacterial genera. Chimpanzees’ (genus: Pan) diet mainly comprise of plants and plants’ produce, with meat contributing only 2% of their diet. Table 1.1 sh ows how different nutrients affect selected bacterial genera. Table 1.1 nutrient bacterial genus Bacteroides Clostridium Fibrobacter carbohydrates digestible (e.g. fruit) – – + non-digestible (e.g. plant fibers) – ++ protein animal-derived + ++ – plant-derived – + fats animal-derived + + – plant-derived – – Adapted from Tomova et al, Frontiers in Nutrition, 6(47), 2019 key + nutrient causes an increase in abundance of bacteria – nutrient causes a decrease in abundance of bacteria shaded boxes mean that information is unknown

4 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination (c) Using your knowledge and information from Fig. 1.1 and Table 1.1, (i) predict the change in proportion for Bacteroides and Fibrobacter in the gut of Homo sapiens and fill in the boxes in Fig. 1.1 with their genus names, and [2] Fibrobacter @ 0.5X or 0.2X Bacteroides @ 2X (ii) comment on the validity of the statement “gut bacteria can act as evidence of evolution”. [4] Valid 1. all four bacteria genera ( fibrobacter, lactobacillus, bacteroides and clostridium) are present in all descendants;; 2. due to descent from common ancestor; 3. female individuals better adapted to environment can survive and reproduce, passing gut bacteria to offspring; OR 4. different abundance of bacteria genera in descendants;; 5. due to different nutrients in diet; 6. different selection pressure resulting in selection for & against of different phenotypes; Not valid - Biodiversity of gut bacteria affected by: 7. o ther than maternal gut microbiota;; 8. other sources include vagina, breast milk, mouth and skin of mothers;; OR 9. external factors of related to individuals;; 10. such as use of antibiotics / lifestyle (including sanitation, caesarean section) / diet;; Max. 2m

5 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination Horizontal gene transfers (HGT) occur frequently in gut bacteria, particularly in response to selection pressures targeting new functions such as antibiotic resistance. Fig. 1.2 shows the HGT rate of antibiotic resistance genes across taxonomic levels of bacteria, and two sets of data, the HGT rate w ithin an individual’s gut and the HGT rate between two individuals’ gut. Adapted from Kent et al, Nature Communications, 2020 Fig. 1.2 (d) (i) Outline how antibiotic resistance genes are transferred between bacteria. [3] 1. Donor / F+ cell synthesizes a sex pilus; 2. Makes contact with recipient / F- cell; 3. Sex pilus retracts; 4. And forms a temporary mating bridge; 5. One strand with antibiotic resistance gene moves to recipient cell; 6. DNA strand then acts as a template; 7. Plasmid in recipient circularises; Max. 3m key within an individual between individuals 100 75 50 25 0 mean 75 percentile 25 percentile 0 percentile 100 percentile between bacterial Species between bacterial Genera between bacterial Families Rate of HGT (per 100 comparisons)

6 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination (ii) Account for one similarity and one difference in the two data sets in Fig. 1.2. [4] Similarity 1. Both mean rates of HGT decreases from between species to between genera / between families;; 2. Sex pilus on donor less able to recognise and bind to target receptors on recipient cells; 3. due to decreasing degree of similarity in nucleotides / shared characteristics at higher taxonomic ranks; Difference 4. Mean HGT rate for within individual is higher than that for between individuals from between species to between families;; 5. Additional step needed for transfer of bacteria; 6. from one individual’s gut to the other individual’s gut;

7 River Valley High School H2 Biology 9744/03 2023 JC2 Preliminary Examination Gut bacteria provide essential health benefits to their hosts. These include nutrient metabolism, drug metabolism and regulating immune homeostasis. Gut bacteria have a role in innate immunity, a